Blood flow measurements in the eye are important to diagnose and monitor progression of diseases and treatment outcomes as a way to reduce the likelihood, or possibly, prevent blindness. The eye is a delicate and sensitive organ that can be damaged by a variety of chronic conditions as well as by acute trauma.
Improved blood flow measurement technology can help to diagnose, monitor and, therefore, possibly prevent blindness. Three of the five most common causes of blindness, macular degeneration, glaucoma, and diabetic retinopathy, are related to the flow of blood in the eye. These conditions must be diagnosed in a timely manner and the treatment must be monitored.
Optical Coherence Tomography (OCT) provides real time images of surface and subsurface structures, and is of particular clinical importance in imaging of the retina. Second generation OCT systems rely on Fourier domain techniques; interference signals are acquired in the optical frequency domain and transformed to the spatial domain. Such techniques include swept source implementations, occasionally referred to as Optical Frequency Domain Imaging (OFDI), or spectrometer-based implementations, referred to equivalently as Spectral Domain OCT or “spectral radar.” A key advantage of Fourier domain techniques is image speed, and commercial speeds of 30 kHz to 100 kHz are now available, and in research labs speeds to 1 MHz (rate for single depth-resolved A-line acquisition) have been reported.
In addition to the structural imaging afforded by OCT, a number of techniques have been proposed for imaging flow, analogous to ultrasound Doppler imaging. Techniques include Color Doppler OCT appropriate for bidirectional flow imaging, and phase-variance or speckle-variance techniques for visualizing the presence of motion. Doppler OCT is discussed in, for example, in U.S. Pat. No. 6,006,128 to Izatt, the contents of which is hereby incorporated herein by reference.
In general, Doppler OCT results are derived from components of flow that co-propagate or counter-propagate with respect to the OCT imaging beam. The process of deriving a physically or physiologically relevant value, such as flow velocity or flow rate requires an assessment of additional parameters, including the angle of flow relative to the interrogating beam and the area of the lumen constraining the flow, and consideration of pulsatility of flow. Error in any of these complementary measures rapidly increases error in quantitative computation of the desired result.
Despite more than a decade of research, there has been no commercialization of a quantitative Doppler OCT system. U.S. Pat. No. 8,244,334 to Huang et al. proposes a dual circumpapillary scan for computing blood flows out of and into the optic nerve head of the eye, but this technique has not been demonstrated to have an accuracy or precision suitable for diagnostic outputs. Furthermore, the eye is served by two circulatory systems: the retinal circulatory system nourishing the inner retina; and the uveal circulatory system, nourishing the outer retina. The circumpapillary approach does not provide information on the uveal circulatory system.